This invention relates to a method for continuously purifying hydrogen or oxygen gas streams.
Hydrogen can be produced by a variety of methods. For example, metals can be reacted with acidic and basic solutions; organic materials can be dehydrogenated with catalysts; organic materials can be reacted with steam (steam reforming); hydrocarbons or carbonaceous materials can be partially oxidized; and electrolyte solutions can be electrolysed. An example of this last method is more fully disclosed in U.S. Pat. No. 3,410,770. In all cases, the evolved hydrogen product is not pure but contains various impurities such as water, oxygen, hydrocarbons, carbon dioxide, carbon monoxide, nitrogen and occasionally particulate matter such as electrolyte salts.
When an extremely pure hydrogen gas stream is desired, the hydrogen gas products of the above processes are typically purified by physiochemcial or chemical means. One such method involves passing the impure gas stream through a barrier of a solid foil of palladium or palladium alloy, normally less than 0.003 inches thick. Because of the atomic lattice structure of the foil, only hydrogen and small quantities of deuterium pass through. By this technique, the purity of a typical incoming gas stream can be upgraded to as high as 99.999999%.
While this purification method can provide extremely pure hydrogen, it has a number of drawbacks in use which render it unsuitable for many applications. First, palladium or palladium alloy foils are expensive. Moreover, because the lattice structure of the palladium is so "tight", the process usually requires an upstream gas pressure on the order of 100 to 300 psig to produce economically feasible amounts of purified gas. Such high upstream pressures usually necessitate an additional upstream compressor, which adds to the expense of the purification process. Additionally, the process is usually carried at an elevated temperature, typically above about 200.degree.C. This also adds to the expense of the purification process. Also, the process cannot be used to purify all hydrogen gas streams, since hydrogen gas streams containing a few materials, such as hydrogen sulfide, poison the foil. Finally, it is necessary to run the process continually, since repeated heating and cooling of the foil cause it to develop severe cracking and fracturing thereby rendering it useless.
Another method developed to purify hydrogen gas streams is based on physical adsorption. This method is especially effective in removing polar molecules such as water and carbon dioxide and is accomplished by simply passing the contaminated gas through a bed of the adsorbent so that the contaminents are retained on the adsorbent surfaces. Various silica and aluminum compounds formed into solid gels have been used for this purpose, and more recently sodium alumino-silicates, commonly referred in the trade as "molecular sieves", have also been used. These compounds are capable of adsorbing as much as one quarter pound of water per pound of molecular sieves before they pass more than one part per million water into the product gas stream. However, a common drawback asociated with molecular sieves is that they are highly effective only with polar molecules. Thus, they cannot be economically used when the gas to be purified contains a high concentration of non-polar molecules, such as oxygen.
It is also well-known that the removal of oxygen from a hydrogen gas stream can be accomplished catalytically. For example, see U.S. Pat. No. 2,582,885. However, a common drawback associated with catalytic removal of oxygen from a hydrogen gas stream is that the catalysts typically employed are subject to deactivation if contacted with minute amounts of water or other poisons. Thus, when used to purify hydrogen gas generated from an electrolytic hydrogen generator, such as the one taught in U.S. Pat. No. 3,410,770, such catalysts are critically deactivated within about 300 hours.
Oxygen gas streams can also be purified by various techniques. For example, the gas may be cooled to liquification and subsequently distilled and/or redistilled until proper purity is obtained. This approach has well-established technology for the production of extremely large quantities of oxygen; however, application of this technology to moderate and small oxygen production is considered economically unfeasible.
Moreover, systems based on physical adsorption principles have also been proposed for the purification of oxygen. When, for example, molecular sieves are employed, lighter molecular weight components evolve from the adsorbent column first, purified oxygen next and finally higher molecular weight and polar materials evolve last. The purified oxygen product must therefore be removed as a "heart cut", and this technique involves by necessity an excessive multiplicity of adsorption/regeneration columns to successfully produce purified oxygen on a continuous basis.
Selective adsorption of oxygen utilizing selective organo-metallic compounds has also been observed, Chemistry of the Metal Chelate Compounds, Martell and Calvin, Prentice-Hall, 1952, pp 336-432, and systems have been constructed to verify that oxygen can be extracted from air. In one such system, the material containing selectively adsorbed oxygen is physically removed to a separate vessel where desorption occurs to yield the oxygen product and the parent adsorbent. The parent adsorbent is subsequently transferred back to the adsorption region and the cyclic process is continued. However, two drawbacks associated with this approach, namely (a) the irreversible oxidation of the adsorbent material thus decreasing the adsorptive properties of the adsorbent and (b) the mechanical fracture of the adsorbent as it undergoes lattice expansion and contraction during the adsorption/desorption cycle, have prevented practical applications of this approach.
Accordingly it is an object of this invention to provide a method and apparatus for removing various impurities from a hydrogen or oxygen gas stream.
It is further object of this invention to provide a method and apparatus for purifying a hydrogen or oxygen gas stream containing both nonpolar and polar contaminants, and especially water.
It is another object of this invention to provide a method and apparatus for removing significant amounts of oxygen and water from a hydrogen gas stream and significant amounts of hydrogen and water from an oxygen gas stream.
It is a particularly preferred object of this invention to provide a simple efficient method and an inexpensive apparatus for purifying the gas products of the apparatus and process shown in U.S. Pat. No. 3,410,770.
It is another object of this invention to provide a simple and inexpensive method and apparatus for producing hydrogen having a purity of as high as 99.9999% and oxygen having a purity of as high as 99.99%.